1. Field of the Invention
The invention relates to a measuring system for nuclear magnetic measuring devices, having a controller, a signal generator for generating and outputting electric excitation signals sa and a signal processor with an input stage and a signal conditioner following the input stage in the signal path of the signal processor for processing the electric measuring signals se effected by the excitation signals sa and being received at the input of the input stage, wherein interfering signals ss also effected by the excitation signals sa and not occurring simultaneously with the measuring signals se are received at the input of the input stage, the signal swing of the measuring signal being less than the signal swing of the interfering signal and the controller determining the instants of time of outputting of the excitation signals.
2. Description of Related Art
The atomic nuclei of the elements having a nuclear spin also have a magnetic moment caused by the nuclear spin. The nuclear spin can be regarded as angular momentum describable by a vector, and accordingly, the magnetic moment can also be described by a vector, which is aligned parallel to the vector of the angular momentum. The vector of the magnetic moment of an atomic nucleus aligns in the presence of a macroscopic magnetic field itself parallel to the vector of the macroscopic magnetic field at the location of the atomic nucleus. The vector of the magnetic moment of the atomic nucleus precesses around the vector of the macroscopic magnetic field at the location of the atomic nucleus. The frequency of precession is the Larmor frequency ωL and is proportional to the magnitude of the magnetic flux density B. The Larmor frequency is calculated according to ωL=γ·B·γ being the gyromagnetic ratio which is maximum for hydrogen nuclei.
Nuclear magnetic measuring devices implement nuclear magnetic resonance measuring methods. These measuring methods influence the precession of atomic nuclei of a medium in the presence of a macroscopic magnetic field by excitation using a controlled magnetic field and evaluate the effects of the influencing. Usually, the precession of the atomic nuclei is influenced by the measuring system, in that electrical excitation signals sa are generated. These electrical excitation signals sa can be converted to magnetic excitation signals in a transmitter coil, which influence the precession of the atomic nuclei. The electric signals usually induced in a sensor coil by the excited precessing atomic nuclei are used as fundamental quantity for the measuring methods. Often only a single coil is used as both transmitter coil and receiver coil.
An example of a nuclear magnetic measuring device is a nuclear magnetic flowmeter for multi-phase media that can measure the flow rate, which is the rate of flow of the individual phases of a medium and the proportions of the individual phases of the multi-phase medium. A prerequisite for measuring a multi-phase medium is that the individual phases of the medium can be excited to distinct nuclear magnetic resonances. Nuclear magnetic flowmeters can be used, e.g., for flow measurement of the multi-phase medium extracted from oil sources. This medium consists mainly of crude oil and salt water in the liquid phase and natural gas in the gaseous phase, wherein all phases contain the hydrogen nuclei necessary for nuclear magnetic resonance and can be excited to different nuclear magnetic resonances.
The measurement of the medium extracted from oil sources can be carried out using test separators. The extracted medium is fed into test separators for a period of time and they separate the individual phases of the medium from one another and determine the proportions of the individual phases of the medium. However, test separators, in contrast to nuclear magnetic flowmeters, are unable to reliably measure crude oil portions less than 5%. Since the proportion of crude oil of each source is steadily decreasing and the proportion of crude oil is already less than 5% in a plurality of sources, it is currently not possible to economically exploit these sources using test separators. In order to also further exploit sources with a very small proportion of crude oil, corresponding exact flowmeters are required for the medium consisting of several phases. In particular, nuclear magnetic flowmeters are a possible solution.
Adaptation of the maximum signal swing processable by the signal processor to the maximum signal swing of the measuring signal se is required for a high accuracy of the measuring system. If the maximum swing of the measuring signals se is less than the maximum signal swing processable by the signal processor, the measurement accuracy decreases. However, the sensor coil does not only receive the measuring signals se effected by the excitation signals sa, but also interference signals ss effected by the excitation signals sa. Common among these interference signals ss is that they do not occur simultaneously with the measuring signals se and the signal swing is greater than the signal swing of the measuring signals se. The excitation signals sa, themselves, belong to the interference signals ss. If only one single coil is used for both transmitting the excitation signals as well as for receiving measuring signals, interference signals ss are the same as excitation signals sa.
Consequently, the maximum signal swing processable by the signal processor is either to be adapted to the maximum signal swing of the interference signal ss or the maximum signal swing at the input of the signal processor is to be limited. Adapting the maximum signal swing processable by the signal processor to the maximum signal amplitude of the interference signal ss, however, decreases the measuring accuracy. Limiting the maximum signal swing at the input of the signal conditioner using a circuit with diodes is known from the prior art. Since, however, the forward voltage of diodes is greater than the signal swing of the measuring signals se, the maximum signal swing that can be processed by the signal processor is to be adapted to the forward voltage of the diodes and not to the maximum signal swing of the measuring signals se. This solution also leads to a reduction of the measuring accuracy.